Rename polyfill to vendor_prefix

cc #906

guide/src/SUMMARY.md

@@ -67,7 +67,7 @@
       - [`static_method_of = Blah`](./reference/attributes/on-js-imports/static_method_of.md)
       - [`structural`](./reference/attributes/on-js-imports/structural.md)
       - [`variadic`](./reference/attributes/on-js-imports/variadic.md)
-      - [`polyfill`](./reference/attributes/on-js-imports/polyfill.md)
+      - [`vendor_prefix`](./reference/attributes/on-js-imports/vendor_prefix.md)
     - [On Rust Exports](./reference/attributes/on-rust-exports/index.md)
       - [`constructor`](./reference/attributes/on-rust-exports/constructor.md)
       - [`js_name = Blah`](./reference/attributes/on-rust-exports/js_name.md)

guide/src/reference/attributes/on-js-imports/polyfill.md

@@ -1,24 +0,0 @@
-# Polyfilling APIs
-
-In JS new APIs often have polyfills via different names in various contexts. For
-example the `AudioContext` API is known as `webkitAudioContext` in Safari at the
-time of this writing. The `polyfill` attribute indicates these alternative
-names.
-
-For example to use `AudioContext` you might do:
-
-```rust
-#[wasm_bindgen]
-extern {
-    #[wasm_bindgen(polyfill = webkitAudioContext)]
-    type AudioContext;
-
-    // methods on `AudioContext` ...
-}
-```
-
-Whenever `AudioContext` is used it'll use `AudioContext` if the global namespace
-defines it or alternatively it'll fall back to `webkitAudioContext`.
-
-Note that `polyfill` cannot be used with `module = "..."` or `js_namespace =
-...`, so it's basically limited to web-platform APIs today.

guide/src/reference/attributes/on-js-imports/vendor_prefix.md

@@ -0,0 +1,24 @@
+# Vendor-prefixed APIs
+
+On the web new APIs often have vendor prefixes while they're in an experimental
+state. For example the `AudioContext` API is known as `webkitAudioContext` in
+Safari at the time of this writing. The `vendor_prefix` attribute indicates
+these alternative names, which are used if the normal name isn't defined.
+
+For example to use `AudioContext` you might do:
+
+```rust
+#[wasm_bindgen]
+extern {
+    #[wasm_bindgen(vendor_prefix = webkit)]
+    type AudioContext;
+
+    // methods on `AudioContext` ...
+}
+```
+
+Whenever `AudioContext` is used it'll use `AudioContext` if the global namespace
+defines it or alternatively it'll fall back to `webkitAudioContext`.
+
+Note that `vendor_prefix` cannot be used with `module = "..."` or
+`js_namespace = ...`, so it's basically limited to web-platform APIs today.